The pathogen that causes plague has been identified in a 4,000-year-old domesticated-sheep carcass, suggesting that livestock helped transmit an early, mysterious form of plague that circulated throughout Eurasia during the Late Neolithic Bronze Age (LNBA), according to a study published yesterday in Cell.
Roughly 5,000 years ago, a mysterious kind of plague distinct from that responsible for bubonic plague spread among people throughout Eurasia before disappearing 3,000 years later, leaving scientists curious about its probable zoonotic source and transmission, said the study team, led by Max Planck Institute researchers in Germany.
Strain previously identified only in ancient humans
The investigators studied sheep bones and teeth excavated at Arkaim, a former site of the Sintashta-Petrovka culture, which was known for cattle, sheep, and horse husbandry on the Western Eurasian Steppe.
Until now, the genome of ancient Yersinia pestis bacteria, which can’t spread via fleas as in bubonic plague, had been identified only in ancient Eurasian humans because of a lack of direct DNA evidence tying animals to human infections in prehistory.
Arkaim “offered us a great place to look for plague clues: they were early pastoralist societies without the kind of grain storage that would attract rats and their fleas—and prior Sintashta individuals have been found with Y. pestis infections,” coauthor Taylor Hermes, PhD, of the University of Arkansas and Max Planck Institute, said in an institute news release.
Many infectious diseases emerged during prehistory, coinciding with animal domestication, which presented opportunities for spillover into people, they added. For example, the domestication of sheep, goats, pigs, and cattle and their interface with people are thought to have driven the emergence of deadly human pathogens causing infectious diseases such as tuberculosis, salmonellosis, and measles.
Sheep, humans probably not main spreaders of disease
A comparison of the ancient Y pestis genome from the sheep with other ancient and modern genomes revealed that the sheep Y pestis genome closely matched one that had infected a human at a nearby site at about the same time.
We collect evidence supporting a scenario where the LNBA lineage, unable to efficiently transmit via fleas, spread from an unidentified reservoir to sheep and likely other domesticates, elevating human infection risk.
“We show that this ancient lineage underwent ancestral gene decay paralleling extant lineages, but evolved under distinct selective pressures, contributing to its lack of geographic differentiation,” the authors wrote. “We collect evidence supporting a scenario where the LNBA lineage, unable to efficiently transmit via fleas, spread from an unidentified reservoir to sheep and likely other domesticates, elevating human infection risk.”
“Collectively, our results connect prehistoric livestock with infectious disease in humans and showcase the power of moving paleomicrobiology into the zooarchaeological record,” they concluded.
But sheep and humans are unlikely to have been the main spreaders of disease, because there are examples of nearly identical LNBA Y pestis genomes at the same time but thousands of kilometers apart, which the researchers say is too far for sick humans or land animals to travel.
Fortunately, the search for pathogens in ancient animal remains is just beginning, because results from past excavations are available for further study. “I think there will be more and more interest in analyzing these collections—they give us insights that no human sample can,” senior author Felix Key, PhD, of Max Planck Institute, said in the release.